The present invention relates to a safety device, in particular for flat-bed knitting machines, having driven cam carriages, which detects the power delivered to the carriages.
For interruptions and malfunctions in the operation of such machines having a drive mechanism, a mechanical safety device having a lever or slides has heretofore been used, which is biased by one or two springs which by their initial tension determine the allowable resistance load limit at which, when it is exceeded, the sled drive mechanism is shut down. Prior art of this kind is described, for example, in the introduction to examined German Patent Application DE-AS No. 21 20 824.
This known safety device not only has the disadvantage that it is relatively sluggish and does not come into action until after a certain amount of delay, because for the safety device to be tripped, this safety device must first travel a certain displacement distance counter to the force of the spring or springs; it also has the disadvantage that a safety device of this kind is capable of being adjusted to only a single very specific threshold value. The power necessary for the cam carriage or carriages over the needle beds is not a constant. Instead, it depends on the number of needles in operation, the number of cams used, the type of operation, the tightness in the loops, and the like. Furthermore, even among individual machines of the same type, it also varies for reasons having to do with manufacturing tolerances. In the known safety device, the threshold value must therefore be set, by trial and error, at least to the maximum expected value for the power of the carriages of the machine type involved. In addition, when flat-bed knitting machines are restarted after being idle for a relatively long period, for example overnight, one problem is that more power is required than after the machine has warmed up. As a rule, this is taken into account by the operators of the machine by adjusting the safety device by hand, et they may fail to reset it after the machine has warmed up.
The invention also relates to a safety device, in particular for flat-bed knitting machines having needle beds, driven cam carriages and piezoelectric oscillation pickups secured to the needle beds which detects impact-like mechanical loads of the needle bed.
In a safety device of this kind, known from examined German Patent Application DE-AS No. 21 20 824, piezoelectric oscillation pickups are used, one of Which is attached to each needle bed. The evaluation of the output signals of these two piezoelectric oscillation pickups per flat-bed knitting machine is effected in parallel fashion with the aid of a voltage divider circuit and a single trigger circuit, connected therewith, for the shutoff device. Once again, the safety device is disadvantageous because each piezoelectric oscillation pickup is set to a quite specific fixed threshold value. The mechanical loads on the needle beds, however, depend on the speed of the carriage assembly, the number of cams, and on the manufacturing tolerances of the machine. Thus once again it is necessary here for the threshold value of the known safety device to be set, by trial and error, above the value that would be expected in the worst case during non-malfunctioning operation. As a result, it is impossible to take into account the fact that the situation can vary from one machine to another and that conditions when a machine starts up after a relatively long idle period differ very greatly from operation in the warmed-up state of the machine.
Furthermore, the present invention relates to a safety device for flat-bed knitting machines with which both the changes that occur in the power of the carriages and the changes that occur in the mechanical load of the needle beds are detected and can lead to a shutoff of the flat-bed knitting machine. None of the aforementioned different malfunction and interruption conditions can be optimally detected by only a part of the safety device in the prior art. For example, European patent 79 386 discloses a safety device which is based on monitoring the change in the carriage speed. In modern drive mechanisms, however, the carriage speed does not change in the event of the aforementioned first disruptions if the carriage is moving sluggishly, and no reduction in speed is to be expected in the second possible disruption mentioned above, in the event of shocks resulting from needle breakage, because the masses of the carriage assembly that are moved are so great that needle or jack butts can be sheared off without affecting the carriage speed.